The present invention relates to a water jet peening apparatus and, in particular, to a water jet peening apparatus suitable for reducing a residual stress in an outer surface of a large-diameter cylindrical structure disposed in a narrow gap space.
In a narrow gap space between a nuclear reactor pressure vessel and its shroud wall in a power generation plant, there are installed various large-diameter cylindrical structures such as a jet pump riser piping, jet pump diffuser and the like that have weldments. In these weldments and also in a part of the structure subjected to thermal stress during welding, there exists a residual tensile stress, which tends to cause a stress corrosion crack afterward. Therefore, there is necessity to relieve such tensile stress or to modify residual tensile stress into a residual compression stress in order to prevent the occurrence of stress corrosion cracking.
A water jet peening method is known as a means for providing a residual compression stress. For example, JPA Laid-Open No. 4- 362124 discloses a water jet peening method wherein a water jet is injected from a jet nozzle disposed opposite to a plate metal material placed in a fluid at the plate metal such that a jet beam containing air bubbles caused by cavitation impinges on the surface of the plate metal. Cavitation air bubbles collapse upon collision of the jet beam, thereby causing water hammering to strike the surface of the plate metal, and thereby providing a compression residual stress in a surface layer thereof.
Further, JPA Laid-Open No. 7-270590 discloses a method of applying a wide-range peening on the surface of a cylindrical structure by causing the water jet to collide on an eccentric position relative to the cylindrical structure.
The above-mentioned prior art method for peening the cylindrical structure by impinging the jet water beam on the surface of the cylindrical structure at the eccentric position thereof is designed to be applied to a group of piping provided in a reactor each having a relatively small diameter of about 100 mm.
On the other hand, inside the reactor pressure vessel, there are installed various types of piping such as a jet pump riser piping that has a diameter of approximately 300 mm, a jet pump diffuser that has a diameter of approximately 500 mm and the like. As for these large-diameter piping, when the jet water beam is impinged from a frontal direction, a side face jet flow cannot be expected to occur in a wide range alike in the case of peening of the flat plate. Therefore, a wide range peening effect cannot be achieved.
Further, the jet pump riser and the jet pump diffuser are located in an annulus section in a gap space between the reactor vessel and the shroud. Because they are located in the narrow gap space, it has been difficult to ensure for an appropriate injection distance and an appropriate angle of incidence of the jet to be achieved.
An object of the invention is to provide for a water jet peening apparatus which is capable of applying an efficient peening work on an external surface of a large-diameter piping for which the effect of the side face flow of the water jet could not have been applied heretofore, and at the same time, which can be installed and operated even in a narrow gap space while ensuring an appropriate injection length and angle of incidence to be achieved.
More specifically, the water jet peening apparatus according to the invention is comprised of a jet nozzle which is supplied with a pressurized fluid and generates a water jet beam in a fluid, which water jet beam contains air bubbles generated by cavitation, wherein the air-bubble containing water jet beam is impinged on a large-diameter cylindrical structure positioned in the fluid for peening of the surface of the large cylindrical structure, and wherein a direction of injection of the water jet beam is at an angle of 45 degrees or smaller relative to a tangential line of the cylindrical structure.
More preferably, the tangential angle is 15 degrees or less.
Further, an outside diameter of the cylindrical structure is preferably larger than 100 mm.
Still further, more preferably, the outside diameter of the cylindrical structure is larger than 200 mm.
Further, a ratio of a length of injection between the nozzle and the cylindrical structure relative to a diameter of the nozzle is preferably 75 to 150.
Still further, preferably, the nozzle is pivotally mounted on a mount base, which has a fixing device to fix its position.
Still more, preferably, the mount base is provided with a plurality of nozzles each having a different direction of injection.
Furthermore, preferably, the fixing device has a nozzle position detection device for sensing and calculating a position of the nozzle from a structure in the rear of the nozzle.
Still further, preferably, the nozzle has a swivel mechanism.